The present invention relates to an endotracheal tube. More specifically, the present invention relates to an endotracheal tube, which permits accurate determination of the pressure exerted by the endotracheal tube's sealing cuff against the tracheal wall.
FIG. 1A shows a prior art endotracheal tube (ETT) 1. FIG. 1B shows a magnified sectional view of ETT 1 taken along line 1B-1B as shown in FIG. 1A. ETT 1 includes a semi-rigid hollow tube 1a, which extends from a proximal end 4 to a distal end 6. Tube 1a is made from poly-vinyl-chloride (PVC). ETT 1 further includes an inflatable balloon, or cuff, 2 mounted near distal end 6. Balloon 2 is sealed to hollow tube 1a at locations 8 and 10 to form an airtight space within the balloon. ETT 1 further includes a central airway lumen 1b, which extends from the proximal end 4 to the distal end 6 of hollow tube 1a. Hollow tube 1a further defines a small inflation lumen 12, which extends through the wall of hollow tube 1a. Inflation lumen 12 provides an opening 18 near its distal end within the interior volume of the balloon 2. At location 5, near the proximal end of hollow tube 1a, the inflation lumen 12 is connected to an inflation line, or tube, 14. An air syringe 16, or other suitable air supply, connected to the proximal end of inflation line 14 selectively controls inflation and deflation of balloon 2. FIG. 1A shows balloon 2 in an inflated condition.
In operation, the distal end 6 of ETT 1 is inserted into the mouth of an unconscious patient, through the patient's natural airway, until the distal end 6 extends into the patient's trachea. The proximal end 4 remains outside the patient. Balloon 2 is in a deflated condition while distal end 6 is being inserted into the patient. After distal end 6 has been positioned within the trachea, balloon 2 is inflated (e.g., by syringe 16) until the outer wall of balloon 2 forms a seal with the inner mucosal lining of the trachea. Once such a seal has been established, a ventilator coupled to the proximal end 4 of ETT 1 may be used to apply intermittent positive-pressure ventilation (IPPV) to the patient. During IPPV, medical gasses supplied to the proximal end 4 of ETT 1 by the ventilator effectively forces the gasses through airway lumen 1b and into the patient's lungs. However, if a seal is not established between balloon 2 and the interior lining of the trachea, gas forced out of distal end 6 simply escapes through the space between balloon 2 and the interior lining of the trachea, and out of the patient's mouth, instead of being forced into the patient's lungs.
Balloon 2 is often constructed from a relatively inelastic material, e.g., PVC. Such inelastic balloons in their inflated condition rarely fit the tracheal diameter exactly. For example, if a patient's trachea is smaller than the expanded size of the balloon, the balloon forms wrinkles at the interface of the balloon and the inner wall of the trachea resulting in an imperfect seal. For example, during long term placement of the ETT, the wrinkles, or micro-leaks, permit fluid and other material to pass between the inflated cuff and the inner lining of the trachea and into the lungs. If, on the other hand, the expanded balloon is too small for the tracheal diameter, no seal will be achieved between the balloon and the inner lining of the trachea. Hence, in practice, since the tracheal diameter is rarely known precisely, the balloon size is always chosen to be larger than the largest expected tracheal diameter. Micro-leaks with such inelastic cuff materials are therefore inevitable.
A further problem attends the use of such plastic ETT cuffs, as noted by Young et al. in GB2324735. When the cuff is inflated within a patient, the pressure within the cuff, or the “intra-cuff pressure”, can be functions of:                1. resistance of the cuff material to stretching;        2. resistance of the tracheal wall to expansion of the cuff; or        3. a mixture of both factors.        
The intra-cuff pressure may be easily measured by, for example, a pressure gauge coupled to the inflation line 14. However, although it is easy to measure the intra-cuff pressure, it is not easy to know how much each of the three above factors contribute to that pressure. Clinically, it is of vital importance to prevent the outer wall of the cuff from applying excessive pressure against the delicate inner lining of the tracheal wall. For convenience of exposition, the term “mucosal pressure” will be used herein to refer to the pressure applied by the outer wall of the inflated cuff to the inner lining of the trachea. If the mucosal pressure is too high, the trachea may become dilated and/or circulation may be cut off in the trachea, which may lead to necrosis of the tissue. In general, the mucosal pressure should be kept below a pressure of thirty centimeters of water. Excessive mucosal pressure caused by over inflation of the cuff can result if there is no feedback to the clinician about the intra-cuff pressure. Additionally, even if the intra-cuff pressure is known, the mucosal pressure generally remains unknown.
In order to overcome this problem, Young et al. in GB2324735, teach the use of a cuff made of a more elastic material such as latex or silicone. An important characteristic of elastic materials such as latex or silicone is that when a sheet of either material is stretched, a point is reached after which the material provides no further resistance to further stretching. When a balloon or cuff formed from elastic material such as latex or silicone is inflated, the intra-cuff pressure initially increases as the volume of the inflated cuff increases. However, with continued inflation the cuff material eventually reaches the point at which it offers no further resistance to stretching. After this point, continued inflation of the cuff causes further expansion of the cuff without a corresponding increase in intra-cuff pressure. In other words, when such an elastic cuff is inflated, the intra-cuff pressure increases initially but then reaches a plateau, and further inflation increases the cuff's volume without causing the intra-cuff pressure to exceed the pressure plateau.
FIG. 2A graphically illustrates the inflation characteristics of an elastic cuff made of latex or silicone. As the volume of gas introduced into the cuff increases from zero to value C, the intra-cuff pressure increases from zero to value A. However, once an intra-cuff pressure of A is achieved, further inflation increases the volume of the expanded cuff, at least to the value D, without raising the intra-cuff pressure. Accordingly, the level A is a pressure plateau. Continued inflation to expand the volume of the balloon beyond value D may eventually cause additional increases in intra-cuff pressure and a final bursting of the balloon. However, the pressure plateau A is not exceeded when the volume is in the range between values C and D.
Young et al. in GB 2324735 teach constructing the cuff of an ETT such that it reaches its pressure plateau before it has expanded sufficiently to circumferentially contact the tracheal walls (i.e., before it has expanded sufficiently to cause contact between the cuff and the inner lining of the trachea along the entire circumference of the trachea). Since the pressure plateau for the cuff is a known constant, when the balloon is inflated to the pressure plateau before it circumferentially contacts the tracheal wall, any additional increase in the intra-cuff pressure (i.e. increase in the pressure within the cuff beyond the pressure plateau), will be caused by contact between the balloon and the trachea (i.e. by the tracheal wall resisting additional expansion of the balloon). Thus, the mucosal pressure can be accurately determined by subtraction (i.e., under these conditions, the mucosal pressure equals the difference between the current intra-cuff pressure and the pressure plateau). Determination, or monitoring, of the mucosal pressure enables avoidance of potentially damaging mucosal pressures.
FIG. 2B graphically illustrates measurement of the mucosal pressure for a latex cuff. FIG. 2B shows the inflated cuff reaching its pressure plateau A before the volume of the inflated cuff is sufficiently large to cause circumferential contact with the trachea. Circumferential contact is achieved at volume value T after which additional increases in intra-cuff pressure are attributable to the inner lining of the trachea resisting further expansion of the cuff. Once circumferential contact is achieved, additional inflation of the cuff causes the intra-cuff pressure to increase from value A to value B along the generally linear pressure-volume curve x. The pressure-volume curve w, generated by subtracting the value A of the pressure plateau from curve x, represents the mucosal pressure. It should be noted that the mucosal pressure is zero until circumferential contact between the cuff and the inner lining of the trachea is achieved.
The volume axis shown in FIG. 2B could alternatively be represented in terms of the diameter of the inflated cuff. To reliably use the above-described method for measuring the mucosal pressure, the cuff should have the following characteristics. The diameter of the inflated cuff corresponding to volume C should be smaller than the smallest expected tracheal diameter (this insures that the cuff reaches its pressure plateau prior to making circumferential contact with the inner lining of the trachea). Also, the diameter of the inflated cuff corresponding to volume D should be larger than the largest expected tracheal diameter (this insures that the inflated cuff makes circumferential contact with the trachea before unrestricted inflation of the cuff could cause the intra-cuff pressure to exceed the pressure plateau). Also, the diameter of the inflated cuff corresponding to volume D should be sufficiently larger than the largest expected tracheal diameter to permit cuff 2 to form a seal (e.g., with a mucosal pressure of 30 centimeters of water) with the largest expected trachea prior to reaching volume D.
Since the inner diameter of the human trachea is relatively small (e.g., from about 1.5 to about 2.5 centimeters in an adult), it is generally difficult to construct the cuff of an ETT such that its diameter, when the pressure plateau is initially reached, is reliably smaller than the smallest expected tracheal diameter. Latex however has several advantageous features that suggest its use as a cuff material. For example, one way to reduce the diameter of a latex cuff at which the pressure plateau is reached, and thereby attempt to ensure that the pressure plateau is reached prior to achieving circumferential contact between the cuff and the inner lining of the trachea, is to longitudinally pre-stretch the latex cuff prior to attaching it to the ETT as taught in Young et al. in GB 2324735. Additionally, latex has been shown to provide a superior seal against the trachea as compared with conventional more inelastic materials since no longitudinal wrinkles are formed in the cuff material that would allow foreign matter to pass through the cuff seal and thereby enter the lungs.
However, the use of latex in medical environments and for medical devices has become increasingly scrutinized, because many people experience an allergic reaction to latex material. The potential allergic reaction can be further complicated by the possibility that the patient may be on a respirator and is possibly in an immuno-compromised state. Additionally, latex material tends to degrade more rapidly than other medical grade material. Thus, it is advantageous to find another material possessing similar characteristics to latex but lacking its allergenic potential and limited shelf-life.
Silicone has been suggested as a suitable cuff material for ETTs. However, unlike latex, silicone does not adhere well to other plastic materials, such as PVC. For at least this reason, ETTs made using a PVC tube and a silicone balloon have not been used in the prior art.
One solution is to make both the cuff and the tube out of silicone. However, this has disadvantages that have not been overcome in the prior art. For example, since silicone is less stiff than for example PVC, a silicone endotracheal tube requires a greater wall thickness than a tube made of another material. Since the inner diameter of the tube is generally determined by the desired airflow characteristics of the tube, the larger wall thickness has the disadvantage of necessitating a larger outer diameter. Because the tube has a larger outer diameter, it is more difficult to have a cuff attached to the tube reach its pressure plateau prior to circumferentially contacting the wall of the trachea. If a silicone tube with thinner wall thickness is used, the tube tends to collapse in either the tubular portion proximal the cuff or at the cuff itself. In either case, if the tube collapses, there is a possibility that the patient will not receive the medical gasses from the ventilator.
Accordingly, there remains a need for an ETT having a cuff with the advantageous features of latex (e.g., providing ability to measure mucosal pressure and providing superior seal) without the disadvantages of latex (e.g., allergic potential and limited shelf life).